1. Industrial Field of Utilization
The present invention relates to a recording apparatus using an on-demand ink jet type recording head, and more particularly to an ink jet type recording head having a driving circuit for forming ink drops at rapid repetition rate.
2. Related Art
An on-demand ink jet type recording head is constituted by a nozzle plate in which a plurality of nozzle openings are formed in one and the same substrate and a spacer for forming pressure generating chambers communicating with the respective nozzle openings so that the pressure generating chambers are expanded/contracted in accordance with print timing signals to thereby perform suction/ejection of ink into/from the pressure generating chambers.
FIG. 1 shows one example of a known ink jet type recording head, and in FIG. 1 the reference numeral 1 represents a nozzle plate having nozzle opening arrays 3, 3, 3 . . . each of which is provided with nozzle openings 2, 2, 2 . . . formed at a predetermined pitch, for example, 180 DPI.
The reference numeral 4 represents a spacer which is to be disposed between a vibration plate 5, which will be described by and by, and the nozzle plate 1, in which spacer through hole arrays 6, 6, 6 . . . for forming reservoirs (not shown), or pressure generating chambers, corresponding to the nozzle arrays are formed in positions corresponding to the nozzle opening arrays, 2, 2, 2 . . .
The reference numeral 5 represents a vibration plate which forms the pressure generating chambers by facing the nozzle plate 1 with the spacer 4 interposed. The vibration plate 5 is disposed so as to be in contact with the tops of piezoelectric vibrators 8, 8, 8 . . . of piezoelectric vibrator units 7, 7, 7 . . . , which will be described later, to thereby contract/expand the pressure generating chambers in response to the expansion/contraction of the piezoelectric vibrators 8, 8, 8 . . .
The reference numeral 9 represents a substrate provided with unit reception holes 10, 10, 10 . . . for receiving the vibrator units 7, 7, 7 . . . so as to expose the free end sides of the piezoelectric vibrators 8, 8, 8 . . . , and an ink supply port 11 for supplying ink from an ink tank into the reservoirs. On the surface of the substrate 9, the vibration plate 5, the spacer 4 and the nozzle plate 1 are positioned and fixed by a frame body 12 which acts also as an electrostatic shield so as to be assembled into a recording head body, so that pressure generating chambers 15 are formed by the spacer 4, the nozzle plate 1 and the vibration plate 5, as shown in FIG. 2, the chambers being supplied with ink from reservoirs 17, 17 through ink supply ports 16, 16.
FIG. 3 shows a driving signal generating circuit suitable to drive the above-mentioned recording head. In FIG. 3, the reference numerals IN.sub.1 and IN.sub.2 represent a print preparation signal input terminal and a print signal input terminal to which a pulse-shaped charge signal P.sub.c as a print preparation signal and a pulse-shaped discharge signal P.sub.d as a print signal are respectively applied in accordance with a print timing signal as shown in FIG. 4A.
The reference numeral 21 represents a level adjusting transistor which has a base electrode connected to the input terminal IN.sub.1 and a collector electrode connected to a base electrode of a first switching transistor 22. Emitter and collector electrodes of the first switching transistor 22 are connected to a power source terminal V.sub.H through a time constant adjusting resistor 23 and to the ground through a time constant adjusting capacitor 24 respectively. The reference numeral 25 represents a constant current control transistor which has an emitter electrode connected to the power source terminal V.sub.H, a collector electrode connected to the collector electrode of the level adjusting transistor 21, and a base electrode connected to the power source terminal V.sub.H through the time constant adjusting resistor 23.
On the other hand, a second switching transistor 26 has a base electrode connected to the input terminal IN.sub.2, a collector electrode connected to the time constant adjusting capacitor 24, and an emitter electrode connected to the ground through a second time constant adjusting resistor 27.
The reference numeral 28 represents a constant current control transistor having a collector electrode connected to the input terminal IN.sub.2, an emitter electrode connected to the ground, and a base electrode also connected to the ground through the second time constant adjusting resistor 27.
The reference numerals 29, 30, 31 and 32 represent transistors constituting a current buffer for amplifying a current at the time of charging and discharging the capacitor 24. In the illustrated embodiment, the transistors 29 and 30, and 31 and 32 are Darlington-connected to have enough current capacitance to drive piezoelectric vibrators of the ink jet recording head to be driven.
The operation of the thus configured driving signal generating circuit will be described. If the recording head moves by a unit distance, a print timing signal (FIG. 4A) for forming a dot is generated from a host. A charge signal P.sub.c (FIG. 4B) of having a pulse width T.sub.c is generated in synchronism with the print timing signal. This pulse width T.sub.c is set to correspond to a sufficient time to allow ink to enter into a pressure generating chamber if the piezoelectric vibrator used is of a d31 type in which the vibrator is contracted by charging. If this signal is supplied to the input terminal IN.sub.1, the level adjusting transistor 21 is turned on, and hence the first switching transistor 22 is also turned on. Consequently, the power source voltage of the power source terminal V.sub.H is applied to the capacitor 24 through the time constant adjusting resistor 23 so that this capacitor 24 is charged with a time constant depending on the resistor 23 and the capacitor 24.
The time constant adjusting resistor 23 is connected at its opposite ends to the constant current control transistor 25 so that the terminal voltage across the resistor 23 is maintained to the voltage between the base and emitter electrodes of the transistor 25 and the current flowing into the capacitor 24 becomes constant without changing over time. As a result, the leading edge gradient .tau.1 of the terminal voltage (V) of the capacitor 24 can be expressed by the following equation: EQU .tau.1 =V.sub.BE1 /(R.sub.1 .times.C.sub.1)
where R.sub.1 represents the resistance of the resistor 23, C.sub.1 represents the capacitance of the capacitor 24, and V.sub.BE1 represents the base-emitter voltage of the constant current transistor 25. The pulse width P.sub.wc of the charge signal P.sub.c is set to a sufficient time to charge the capacitor 24 up to the voltage V.sub.0 of the power source terminal V.sub.H.
After the time corresponding to the pulse width T.sub.c of the charge signal P.sub.c has thus passed, the terminal voltage of the capacitor 24 is increased up to the power source voltage V.sub.0. The charge signal P.sub.c is switched to an L level at this time, so that the level adjusting transistor 21 is turned off, and hence the first switching transistor 22 is also turned off. As a result, the capacitor 24 keeps the voltage .tau..times.T.sub.c =V.sub.0.
If a discharge signal P.sub.d (FIG. 4C) as a print signal is supplied to the terminal IN.sub.2 when a predetermined time P.sub.wh has passed since the moment the charge signal P.sub.c was turned off, the second transistor 26 is turned on to form a loop for discharging the charges of the capacitor 24.
As a result, the charges accumulated in the capacitance C.sub.1 are discharged through the time constant adjusting resistance R2 of resistor 27. At the same time, the constant current control transistor 28 is turned on so that the terminal voltage of the second time constant adjusting resistor 27 is made equal to the base-emitter voltage V.sub.BE2 of the transistor 28 by the same effect as the above-mentioned effect of the first constant current control transistor 25, so that the terminal voltage (V) of the capacitance C.sub.1 drops with a constant gradient.
That is, the trailing edge gradient .tau.2 can be expressed by the following equation: EQU .tau.2=-V.sub.BE2 /(R.sub.2 .times.C.sub.1)
where R.sub.2 represents the resistance of the second time constant adjusting resistor 27, C.sub.1 represents the capacitance of the capacitor 24, and V.sub.BE2 represents the base-emitter voltage of the constant current transistor 28. The pulse width P.sub.wd of the discharge signal P.sub.d is set to a sufficient time to discharge the capacitor 24 down to zero potential.
The voltage changing at a predetermined leading edge speed and a trailing edge speed depending on the time constant adjusting resistors 23 and 27 and the capacitor 24 in such a manner as described above is amplified by the transistors 29 and 30, and 31 and 32 respectively constituting a current buffer, and applied to the piezoelectric vibrators 8, 8 (FIG. 2).
In a thus configured driving signal generating circuit applied to a pull-dotting system ink jet recording head, if a charge signal P.sub.c is applied to the terminal IN.sub.1 at the time T1 synchronously with a print timing signal a constant current flows into the piezoelectric vibrator 8, and the terminal voltage (FIG. 4D) of the piezoelectric vibrator 8 increases at a constant rate. The vibration plate 5 contracts at a constant rate correspondingly so as to be displaced downward in FIG. 2. The volumes of the pressure generating chambers 15, 15 are expanded correspondingly and negative pressure is generated in the pressure generating chambers 15, 15 so that the ink in the reservoirs 17, 17 flows into the pressure generating chambers 15, 15 through the ink supply ports 16, 16, and at the same time the menisci of the nozzle openings 2, 2 are pulled into the pressure generating chambers 15, 15.
The menisci move toward the nozzle openings because of surface tension after they are pulled into the pressure generating chambers 15, 15 to some extent (FIG. 4E).
At a point of time (T.sub.2) when the time corresponding to the pulse width P.sub.wc of the charge signal P.sub.c has passed and charging the piezoelectric vibrator 8 has been finished, the terminal voltage of the piezoelectric vibrator 8 is in a so called hold state where it is held at the power source voltage V.sub.0. Therefore, if a discharge signal P.sub.d is applied at a point of time (T.sub.3) when a given hold time P.sub.wh has passed, the charges of the piezoelectric vibrator 8 are discharged at a constant rate so that its terminal voltage is decreased at a constant rate (FIG. 4D). Thus the pressure generating chambers 15, 15 contract to eject ink as ink drops from the nozzle openings.
The charges of the piezoelectric vibrator 8 are perfectly discharged at a point of time (T.sub.4) when the drops of ink are ejected and the time corresponding to the pulse width P.sub.wd of the discharge signal has passed.
On the other hand, the meniscus is formed in the pressure generating chamber 15 because ink corresponding to the volume of the ink drop is discharged from the pressure generating chamber 15, and the meniscus produces residual vibrations with an inherent vibration period depending on the physical properties of the ink, the size of the pressure generating chamber 15, and the size of the member constituting the pressure generating chamber 15. Therefore, as shown in FIG. 4E, the meniscus repeats movement toward the outside of the nozzle opening or toward the pressure generating chamber side.
In order to prevent the influence of such vibration of the meniscus, time P.sub.wt, which is necessary to attenuate the vibration to a sufficient extent not to give any influence to the formation of a dot, is established, or the pulse width P.sub.wc of the charge signal P.sub.c and the hold time P.sub.wh are elongated to a sufficient degree.
However, the speed of printing is reduced if such a pause period P.sub.wt is established or the charge pulse width P.sub.wc and the hold time P.sub.wh are elongated. Alternatively, the position of the meniscus at the time of ejection can be changed with a driving frequency if typing is performed at a high speed. In this case, unlike the above- mentioned case in which the meniscus is in a stationary state, the position of the meniscus at the time of output of a print timing signal is, for example, in the pressure generation chamber side, so that there occurs a new problem that the quality of printing varies depending on the frequency.
That is, if a print timing signal is outputted when the meniscus, which vibrates due to the residual vibration of the piezoelectric vibrator, moves toward the nozzle opening, negative pressure caused by the expansion of the pressure generating chamber 15 produces a force to move the meniscus toward the pressure generating chamber. Such a force is however canceled by the force of the meniscus per se to move to the outside of the nozzle opening due to the above-mentioned residual vibration. The result is that the influence of the negative pressure is reduced as much as possible, and the meniscus is returned to the nozzle opening side at once. If, then, the charges of the piezoelectric vibrator are discharged at a constant rate so that the piezoelectric vibrator expands, an ink drop is formed in such a state that the meniscus is positioned in the nozzle opening side as much as possible. Accordingly, it is possible to obtain a necessary volume of the ink drop. The flying speed, however, generally becomes low.
Conversely, if a print timing signal is outputted when the meniscus, which vibrates due to the residual vibration, is moving toward the pressure generating chamber, the movement of the meniscus caused by the residual vibration falls on the movement of the meniscus toward the pressure generating chamber caused by the negative pressure produced by the expansion of the pressure generating chamber, so that the meniscus moves deeply into the pressure generating chamber and the return of the meniscus toward the nozzle opening is delayed. If, then, the charges of the piezoelectric vibrator are discharged at a constant rate so that the piezoelectric vibrator expands, an ink drop is formed while the meniscus is drawn into the pressure generating chamber away from the nozzle opening and the ink drop is rendered small in volume even though the flying speed becomes high.
Thus the size and speed of the formed ink drop vary greatly depending on the position of the meniscus, even if the piezoelectric vibrator is driven with the same energy. As a result, dots formed on a recording medium vary in size so that the printing quality is lowered.
Also, in such a configuration in which the pressure in the pressure generating chambers is changed correspondingly to the print timing signals, vibrations from mechanical structures of the pressure generating chambers per se and hydrodynamic vibrations of ink per se are generated, thereby causing vibrations of menisci in the vicinity of the respective nozzle openings such that the menisci reciprocate between the nozzle openings and the respective pressure generating chambers after formation of ink drops.
As a result, even if the same pressure change is generated in each pressure generating chamber, the ejected ink drop varies in its size and flying speed depending on the positional relationship between the associated nozzle opening and the meniscus formed in the vicinity of the nozzle opening, resulting in a problem that variations are caused in printing quality.
In order to solve such a problem, it is possible to consider a technique whereby successive ink drop formation is performed only after the vibration of the meniscus, caused after the formation of the preceding ink drop, is reduced to such an extent as to have no influence on the printing quality. This technique however has a problem in that the printing speed is greatly reduced due to the waiting time needed until the vibration of the meniscus is suppressed.